The cellular mechanism of action of phorbol ester (PE) tumor promoters is being investigated in normal human peripheral blood lymphocytes as well as in a mouse T cell line. Both of these cell populations respond to PEs with production of T cell growth factor (TCGF). Specific, saturable, high affinity PE receptors which may mediate the PE stimulated TCGF production have been identified in both systems as judged by appropriate time courses, dose response curves, and structure activity relationships for binding and TCGF production. Current studies are directed at localizing and characterizing the receptor and identifying steps occurring subsequent to PE binding and leading to TCGF production.